1. Field of the Invention
The present invention relates to a color image reading apparatus for reading a color image recorded on, e.g., a color negative film.
2. Related Art
The present inventor has proposed an apparatus for photoelectrically reading a color image by using a CCD image sensor for use in a color copying machine, a color facsimile apparatus, or the like in each of U.S. application Ser. Nos. 544,240, 061,042 U.S. Pat. No. 4,734,762, and 193,227 U.S. Pat. No. 4,891,690. The present inventor has also proposed an apparatus for reading a color image recorded in a color film on each of U.S. application Ser. Nos. 013,731 U.S. Pat. No. 4,933,983 and 119,906 aban. 9/12/89.
In such a color image reading apparatus, unlike in a conventional monochromatic image reading apparatus, an image must be read with not only a high light intensity but also a good color balance. For this purpose, various image processing operations must be performed.
In particular, reading of a color image recorded on a color negative film has the following problems. That is, a color negative film is-generally subjected to processing called an orange mask, i.e., a negative image is recorded on an orange-colored base film. Therefore, it is impossible to visually discriminate disturbance in a color balance of a negative film caused by light source conditions and the like upon photography. A color negative film reading apparatus, i.e., a color photograph printing apparatus is conventionally known as an apparatus for correcting a color imbalance to obtain a positive print having a proper color balance.
This color photograph printing apparatus measures an average transmission density of a film for each of R, G, and B by utilizing Evans' theorem, i.e., an empirical principle (U.S. Pat. No. 2,571,697) in which an achromatic color is obtained by averaging colors of a camera subject on an entire frame, compares the measurement value with an average transmission density of a standard negative film, i.e., a negative film photographed outdoors on a fine day with a proper exposure value, and adjusts an exposure time t.sub.i upon printing in accordance with equation (1), thereby obtaining a positive film having a proper color balance (Japanese Patent Publication Nos. 60-46693 and 61-10817); EQU logt.sub.i =.alpha..sub.i D.sub.i +.beta..sub.i ( 1)
where
t.sub.i : an exposure time (i=R, G, and B) for each of R, G, and B PA1 D.sub.i : an average transmission density of a negative film to be printed PA1 .alpha..sub.i, .beta..sub.i =constants determined by an average density of a standard negative film, the type of film, properties of printing paper, and the like
A color negative film, however, differs in an orange base density, color development characteristics of a dye, the sensitivity of a dye layer of each of R, G, and B, and the like in accordance with the film manufacturer or the type of a film, i.e., values of the constants .alpha..sub.i and .beta..sub.i differ between films. Therefore, the values of the constants .alpha..sub.i and .beta..sub.i must be determined for each negative film and the predetermined constants must be stored.
In addition, even if a photograph which does not satisfy Evans' theorem, e.g., a photograph of an evening glow or a photograph in which a color balance is intentionally disturbed by using a color filter or the like is printed within an exposure time satisfying equation (1), a desired color is not reproduced in an obtained print.